1. Field of Invention
The present invention relates to the field of providing process data to a client, such as for monitoring an Industrial Automation System.
2. Related Information
Remote monitoring and control of systems and processes have taken many forms. In the past, dedicated lines became the most common form of communication between a Control System and a remote location. Even today, many industry systems prefer to employ dedicated lines because they are secure, meaning that sensitive data, such as proprietary Industrial software code, is not at risk of being intercepted by competitors, and they are reliable, since there are less problems with connectivity and redundancy, for example, in such a direct connection.
For all of the advantages of the dedicated line, this method of connection has limited application because the Control System is, for one thing, not accessible from multiple locations. For this reason, amongst others, Industrial technologists have explored the use of Modems to make access to the Control System possible from different locations While Modems appear to solve the problem of remote connectivity, the actual reality is that these types of systems are much too unreliable, particularly in the Industrial field where information transmission is critical to a manufacturing process, for example. As a result, the practical use of Modems is generally restricted to downloading and uploading data files. In other words, providing any type of control function between locations is rather limited in this type of environment. Further, an end user generally required a customized interface to access the Control System.
With the explosion and subsequent growth of the Internet, the World Wide Web provides a promising, delivery platform that offers an opportunity for organizing and providing Internet data. The Web, a network of documents called sites or pages stored on server computers throughout the world, could provide access any Industrial system anywhere. Moreover, it is conceivable, for example, that data could be sent via hypertext links, for example, on a client server system. In this manner it is possible that a system could b provided that will, not only provide remote access for Industrial systems, but also give each end user the same degree of a user friendly interface with the same universal access to services on the Web.
However, the solution of remote access is not as easily implemented as that. Each Web page usually contains Text, i.e., some type of Multimedia offerings such as Graphic Images, Video, or Audio, and possible Hypertext Links to other documents. And that is an oversimplification of the media needed to support a client server Web Page. In addition, a Browser is required to allow a user to read the pages and interact with the choices associated with it. Normally, the Browser is a graphical software program that sends commands to the Internet Web site and displays whatever information is available on the page. While various Browser programs are commercially available from different manufacturers, it is too simplistic to expect that merely launching a Browser will solve all the connectivity problems and protocol issues required to, for example, access data and control remote Industrial Controllers.
A particular example that frustrates the use of the Internet Network for Critical Applications, such as in the Industrial Control Industry, is that the Internet is based on the fundamental concept of connecting multiple hubs together to form a network of access. Thus, a single Internet Server may handle thousands of general purpose computer connections at a moment through a single connection. The problem with this philosophy, while practical for broadband communication to remote sites, therefore, is that the Internet has no innate ability to differentiate traffic in terms of its purpose or the criticality of its data. Indeed, TCP/IP (Transfer Communication Protocol/Internet Protocol) is based on the Ethernet Protocol developed at Xerox Park, mostly to the credit of Robert Metcalfe, and Ethernet dices the data to be transmitted into packets that are often jammed together with packets from other sources of data to form a transmission stream. The Internet is no longer a network of computers that often shares a single transmission connection
To compound the problem, the Web may be interconnected or may be set up as a point to point link, such as in peer to peer communication. The Internet may, as well, involve both general purpose stations and specialized infrastructure components. For example, there are routers and firewalls that need to be dispensed with. These and other hardware and firmware issues need to be resolved.
Another fundamental principal of the Internet is that the type of personal computer or work station used by the end user to connect to the Web is typically of no regard to the Internet. Communication over the Internet and other networks accepts one of several types of protocols. Protocols such as Internet Protocol (IP) provide for file transfers, electronic mail, and other services. However, in specialized applications, such as Industrial Networks, it is possible that special provisions are required at the user end to interface with the user.
On the other side of the Industrial Connectivity issue, are the Programmable Logic Controllers. These are widely used in Industry and Process Control. Many manufacturers provide factory automation information using Microsoft Windows™ and other systems providing communication networking capabilities. However, these networks are usually slow, are not universally accessible and are limited to monitoring and data exchange.
Control may be implemented using such networks, but since the communication networks are non-deterministic, control is simply not in real time. Perhaps, specialized Industrial Network Systems using proprietary Fieldbus alternatives, but these can be very expensive. For one thing, conversion products are required to allow information carried over those networks to be visible on a general purpose network. Moreover, there are significant installation and other deployment costs associated with the existence of such intermediate devices. Firewalls between the Web server and the application are designed to solve problems of security and are not designed for high performance.
U.S. Pat. No. 6,061,603 discloses an Interface between an Industrial Control System and a Web Browser is known. This System is explained by making reference to FIG. 1.
FIG. 1 shows an overview block diagram of a typical system illustrating the relationship between a user 2 at a remote location and an Internet Web site 4 used for monitoring a Process Control System 6. The user 2 will have a personal computer (PC) 8 having a commercially available Browser 10, such as Netscape Communication's Navigator or Microsoft's Internet Explorer, installed for viewing the contents at the Web site 4 by a monitor 12.
The PC provides a remote Human-Machine Interface (HMI) to the process Control System 6. Various interconnection services are readily available to provide the physical and electrical interconnection from the PC to the Internet 14 itself. The Internet 14 is a collection of independent world wide communication networks that are interconnected to each other and function as a single connectionless entity.
Communication is based on a client-server basis, using a number of established protocols that allow for communication and file transfers between the client and the server. The most widely used protocol is Internet Protocol (IP).
The Web site 4 includes a network interface 16 having an unique Internet address 18, a server 20, and an application program 22. The server 20 acts as the HTTP interpreter which uses TCP in conjunction with IP, through TCP/IP stack 24 to interact with the network interface 16 and the application program 22.
This enables the data transfer between the application program 22 and the user 2 through the Internet 14. The application program provides data from the process Control System 6. This data can be used to monitor the control process by the user 2 at the remote location. The TCP/IP stack 24 enables data transfers over the Internet 14 between the user 2 and the Web site 4 as required for the various layers specified by the IP protocol.
The user 2 can connect to the Internet 14 using one of a number of Internet service providers and will enter the address of the Web site 4 when connected. The Web site 4 will display a home page which may contain text, some type of multimedia offerings such as graphic images, video, or audio, and possible hypertext links to other documents.
The Browser 10 will allow the user 2 to read the page and interact with the choices associated with it. The Browser 10 will send commands to the Web site 4 which will use the application program 22 to display whatever information is available from the process Control System 6. The Browser 10 functions as a remote human-machine interface or HMI control of the process Control System as will be detailed below.
Another prior art approach is OLE for Process Control (OPC).
At a high level, an OPC server is comprised of several objects: the server, the group, and the item. The OPC server object maintains information about the server and serves as a container for OPC group objects. The OPC group object maintains information about itself and provides the mechanism for containing and logically organizing OPC items.
The OPC Groups provide a way for clients to organize data. For example, the group might represent items in a particular operator display or report. Data can be read and written. Exception based connections can also be created between the client and the items in the group and can be enabled and disabled as needed. An OPC client can configure the rate that an OPC server should provide the data changes to the OPC client.
There are two types of groups, public and local (or ‘private’). Public is for sharing across multiple clients, local is local to a client. There are also specific optional interfaces for the public groups. Within each Group the client can define one or more OPC Items.
The OPC Items represent connections to data sources within the server. An OPC Item, from the custom interface perspective, is not accessible as an object by an OPC Client. Therefore, there is no external interface defined for an OPC Item.
All access to OPC Items is via an OPC Group object that “contains” the OPC item, or simply where the OPC Item is defined. Associated with each item is a Value, Quality and Time Stamp. The value is in the form of a VARIANT, and the Quality is similar to that specified by Fieldbus.
Note that the items are not the data sources—they are just connections to them. For example, the tags in a Control System exist regardless of whether an OPC client is currently accessing them. The OPC Item should be thought of as simply specifying the address of the data, not as the actual physical source of the data that the address references.
An OPC application comprises COM (Communication Objects Model) or DCOM (Distributed Communication Object Model) objects and interfaces implemented by OPC servers.
The distributed component objects model (DCOM) is a protocol that enables software components to communicate directly over a network.
DCOM is an extension of the Component object Model (COM). COM defines how components and their clients interact. This interaction is defined such that the client and the component can connect without the need of any intermediary system component. In today's operating systems, processes are shielded from each other. A client that needs to communicate with a component in another process cannot call the component directly, but has to use some form of interprocess communication provided by the operating system. COM provides this communication in a transparent fashion: It intercepts calls from the client and forwards them to the component in another process. When client and component reside on different machines, DCOM replaces the local interprocess communication with a network protocol.
Network connections are inherently more fragile than connections inside a machine. Components in a distributed application need to be notified if a client is not active anymore, even—or especially—in the case of a network or hardware failure.
DCOM uses a pinging protocol to detect if clients are still active. Client machines send a periodic message. DCOM considers a connection as broken if more than three ping periods pass without the component receiving a ping message. If the connection is broken, DCOM decrements the reference count and releases the component if the count has reached zero. From the pont of view of the component, both the benign case of a client disconnecting and the fatal case of a network or client machine crash are handled by the same reference counting mechanism.
With DCOM, any component can be both a provider and a consumer of functionality. The same mechanism and features manage communication in both directions, making it easy to implement peer-to peer communication, as well as client/server interactions. In more recent work DCOM is replaced by a more loosely coupling between client and server through messages being sent from clients to server and vice versa.
Although message based communication is used a lack of flexibility and a lack of extensibility is a common disadvantage of the above described prior art approaches. For example if one of the software components is modified or if a new functionality is added the other components which interact with that component need to be adapted correspondingly.
It is therefore an object of the present invention to provide for an improved method of providing process data to a client, and to provide for a corresponding computer program product as well as improved server and client computers of an Industrial Automation System.
The object of the invention is solved by applying the features laid down in the respective independent claims. Preferred embodiments of the invention are given in the dependent claims.
The present invention is particularly advantageous in that it provides for a flexible, open and extensible communication means for client-server communication in an Industrial Automation System. This enables to modify and/or to add functionality to a server or client application without having to change or adapt other system components. This is made possible by the usage of mark up languages like XML or SGML or a similar mark up language as a transport and control format for the client-server communication.
In the following parts of this document the term “XML type message” is used for a document or text or message or parameter containing information formulated using a markup language like XML, SGML or alike. The term “XML-type” is used as an adjective to specifiy that a text, document, message or parameter is formatted according to a markup language like XML, SGML or other markup languges which enable a structuring of information.
The term “protocol” is used for a defined, specified or standardised sequence of messages, documents, function-calls or method-calls from a client to a server and vice versa.
Extensible means that a client or server can add additional data or meta data to a request or response without breaking compatibility with the protocol.
In accordance with a preferred embodiment of the invention the client computer directs a more or less unspecific request for runtime process data to a server of the Industrial Automation System. In response to this request the server determines the structure and the kind of process data to be provided to the client e.g. according to the request and the state or capabilities of the server.
A server application generates an XML type document with meta data being a description of the structure and/or meaning of the data being sent and containing the process data itself. This XML type document is sent to the client computer and is interpreted by an application program of the client computer. It is an important advantage that the content and/or structure of the data contained in the response message is not defined or is only partly defined by the requesting client. Rather, it is defined dynamically by the server both in structure as well as in content.
Furthermore it is possible, that the data contained in the response message and described by the meta data of the response message can itself be a text formulated using a markup language like XML and adding another level of flexibility and extendability.
In accordance with a further preferred embodiment of the invention the client computer makes the unspecific request mentioned above in the form of an XML type message.
In accordance with a further preferred embodiment of the invention the client computer makes a more specific request in the form of an XML type messsagewhich contains a subscription condition for process data.
For example if a subscription condition is met a refresh operation is to be performed and an XML type response message shall be sent. In principle any condition can be coded into the XML type message provided by the client to the server. Typical conditions are a certain cycle time for the data refresh or a change of a process parameter or parameters.
Alternatively or in addition in accordance with a preferred embodiment of the invention the XML type message from the client can also contain (meta) data which is a description or specification (e.g. a structure) of the process data to be provided by the server. This description or specification is extendible and open i.e. not fully specified in the specification of the access protocol. This enables that specific clients can formulate their own description or specification without braking the specification of the protocol and therefore being more flexible.
The server computer provides an acknowledgement to the client computer in which it informs the client if it can fulfil the specification contained in the request document provided by the client computer. If the contrary is the case this is also signalled to the client computer. In one embodiment the client computer modifies the specification contained in the XML document and sends a new XML document to the server computer until an acceptable XML document has been “negotiated” between the client and the server.
In accordance with a further preferred embodiment of the invention the server communicates to the client the specifications which it can support in form of a part of an XML type message. Based on this server information the client formulates the specifications contained in the XML type data request message. This can be done by selecting a subset of the specification supported by the server on the client side.
In accordance with a further preferred embodiment of the invention a certain set of specifications is standardized. Additional customer or manufacturer specific specifications can be added.